1. Field of the Invention
The present invention relates to piston type compressors that convert rotation of a rotary shaft to linear reciprocation of a piston with a driving body such as a swash plate, and more particularly, to pistons used in such compressors.
2. Description of the Related Art
Compressors are employed in air-conditioning systems for vehicles. Piston type compressors are used in such systems. A typical piston type compressor is provided with a driving body, such as a swash plate, to reciprocate pistons. The swash plate is supported by a drive shaft in a crank chamber and converts the rotation of the drive shaft to the linear reciprocation of each piston in an associated cylinder bore. The reciprocation of the piston draws refrigerant gas into the cylinder bore from a suction chamber, compresses the gas in the cylinder bore, and discharges the gas into a discharge chamber.
The typical piston type compressor draws the refrigerant gas from an external refrigerant circuit into a suction chamber by way of the crank chamber. In such a compressor, in which the crank chamber constitutes a portion of a refrigerant gas passage, the refrigerant gas from the external refrigerant circuit passing through the crank chamber sufficiently lubricates various parts in the crank chamber, such as the piston and the swash plate, with the lubricating oil suspended in the gas.
There is also a type of compressor that draws in refrigerant gas from an external refrigerant circuit without having the gas flow through its crank chamber. In such a compressor, the driving plate, or swash plate, is supported so that it inclines with respect to the drive shaft. The inclination of the swash plate changes in accordance with the difference between the pressure in the crank chamber and the pressure in the cylinder bores. The displacement of the compressor varies in accordance with the inclination of the swash plate. The difference between the pressure in the crank chamber and the pressure in the cylinder bores is changed, for example, by adjusting the pressure in the crank chamber using a control valve. Since the pressure of the crank chamber is adjusted to control the inclination of the swash plate in such type of compressor, the crank chamber is not included in the suction passage. Therefore, the various parts in the crank chamber are lubricated mainly by lubricating oil that is included in blowby gas. Blowby gas refers to the refrigerant gas in the cylinder bore that leaks into the crank chamber through the space defined between the outer surface of the piston and the wall of the associated cylinder bore when the piston compresses the refrigerant gas in the cylinder bore.
The amount of blowby gas, or lubricating oil, supplied to the crank chamber is determined by the dimension of the clearance defined between the outer surface of the piston and the wall of the cylinder bore. Accordingly, it is necessary to increase the dimension of the clearance to supply a sufficient amount of lubricating oil for satisfactory lubrication of the various parts in the crank chamber. However, a large clearance between the piston and the cylinder bore degrades the compressing efficiency of the compressor.
To cope with this problem, compressors such as that shown in FIG. 8 are known in the prior art. The compressor has a swash plate 100. The swash plate 100 is mounted on a drive shaft 104 in a crank chamber 103, which is provided between the cylinder block 101 and the front housing 102, and supported so as to rotate integrally with the shaft 104. Single-headed pistons 105 are each accommodated in a cylinder bore 101a, which is provided in the cylinder block 101. A skirt 105a projects from the rear side of each piston 105 (to the left as viewed in FIG. 8) toward the crank chamber 103. The skirt 105a is operably connected to the swash plate 100 by a pair of shoes 106. Each shoe 106 is slidably clamped between the skirt 105a and the swash plate 100. The rotation of the drive shaft 104 is converted to the linear reciprocation of the piston 105 in the cylinder bore 101a by means of the swash plate 100 and the shoes 106.
An annular groove 107 extends along the outer surface of each piston 105. Lubricating oil applied to the wall of the cylinder bore lOla is collected in the groove 107 and guided toward the crank chamber 103 during reciprocation of the piston 105. The lubricating oil lubricates the connecting portion between the swash plate 100 and the piston 105. Accordingly, in compressors that employ pistons having such structure, the various parts in the crank chamber may be satisfactorily lubricated without enlarging the dimension of the clearance between the piston and the cylinder bore, or without reducing the compressing efficiency of the compressor.
As shown in FIGS. 8 and 9, the skirt 105a of the piston 105 has an arched surface 105b, which is defined on the surface facing the inner surface of the front housing 102. The arched surface 105b slides against the inner surface of the front housing 102. The radius of curvature of the arched surface 105b is the same as that of the inner surface of the front housing 102. When the piston 105 reciprocates, the arched surface 105b slides against the inner surface of the front housing 102 and prevents the piston 105 from rotating about its axis.
The arched surface 105b extends along the entire width of the skirt 105a that faces the inner surface of the front housing 102. However, it is difficult to accurately machine the entire arched surface 105b so that it has the same radius of curvature as the inner surface of the front housing 102.
Furthermore, the entire arched surface 105b, which extends for a wide range, slides against the inner surface of the front housing 102. Thus, when the piston 105 moves from the top dead center position to the bottom dead center position, the lubricating oil on the end face of the skirt 105a and the lubricating oil that collects at the bottom of the crank chamber 103 is dispersed toward the left, as viewed in FIG. 8. The lubricating oil is not guided to the connecting portion between the piston 105 and the swash plate 100. Accordingly, this oil is not used efficiently, and the connecting portions between the pistons 105 and the swash plate 100 are not lubricated to the degree that is desirable.